Includes referencesPolitical dynasties have long been present in democracies, raising concerns that inequality in the distribution of political power may reflect imperfections in democratic representation. However, the persistence of political elites may simply reflect differences in ability or political vocation across families and not their entrenchment in power. We show that dynastic prevalence in the Congress of the U.S. is high compared to that in other occupations and that political dynasties do not merely reflect permanent differences in family characteristics. On the contrary, using two instrumental variable techniques we find that political power is self-perpetuating: legislators who hold power for longer become more likely to have relatives entering Congress in the future. Thus, in politics, power begets power.